As a method for cleaning nitrogen oxide (NOx) in an exhaust gas of an internal combustion such as a car, a urea SCR (Selective Catalytic Reduction) method has been developed. The urea SCR method is a method for reducing NOx by ammonium that is generated by adding urea to an SCR catalyst, and an ammonium gas sensor is used for measuring whether or not an ammonium concentration for reducing NOx is in an appropriate amount.
U.S. Patent Application Publication No. 2007/0045114 (FIG. 1, paragraph 0032, claim 1, hereinafter, Patent Document 1) describes a first related art gas sensor. For example, the first related art ammonium gas sensor is an electromotive force type sensor that detects an ammonium concentration based on an electromotive force between a reference electrode and a reaction electrode formed on a surface of an oxygen ion conductor. Specifically, the first related art gas sensor is a sensor in which a reaction electrode 21 and a reference electrode 22 are opposed to each other via a solid electrolyte layer 20 made from zirconium, and the reaction electrode 21 is disposed directly under a protection layer 23 that is provided on a sensor surface and made from a porous material (see Patent Document 1). In Patent Document 1, as a composition of the reaction electrode 21, oxide represented by AxMyOz such as BiVO4 (M represents vanadium, tungsten, or molybdenum) is described.
Also, in general gas sensors, in order to prevent poisoning of a detection electrode by a component (e.g. oil component, P, Si, and the like) in a gas to be measured, a surface of the detection electrode is covered with a protection layer made from a porous material. Japanese patent unexamined patent application publication No. JP-A-2001-174434 (Hereinafter, Patent Document 2) describes a second related art gas sensor. For example, Patent Document 2 discloses a technique of adding a zirconium oxide component that is a component of a solid electrolyte serving as an underlying layer to the protection layer in order to improve a contact of the protection layer.